1. Field of the Invention
The invention relates to light emitting devices (LED) packages and, more particularly, to white LED and multiple LED packages with scattering particles.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. Typically, wire bonds are used to apply a bias across the doped layers, injecting holes and electrons into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED. A typical high efficiency LED comprises an LED chip mounted to an LED package and encapsulated by a transparent medium. The efficient extraction of light from LEDs is a major concern in the fabrication of high efficiency LEDs.
LEDs can be fabricated to emit light in various colors. However, conventional LEDs cannot generate white light from their active layers. Light from a blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; See also U.S. Pat. No. 5,959,316 to Lowrey, “Multiple Encapsulation of Phosphor-LED Devices”]. The surrounding phosphor material “downconverts” the energy of some of the LED's blue light which increases the wavelength of the light, changing its color to yellow. Some of the blue light passes through the phosphor without being changed while a portion of the light is downconverted to yellow. The LED emits both blue and yellow light, which combine to provide a white light. In another approach light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes.
A common type of LED packaging where a phosphor is introduced over an LED is known as a “glob-in-a-cup” method. An LED chip resides at the bottom of a cup-like recession, and a phosphor containing material (e.g. phosphor particles distributed in an encapsulant such as silicone or epoxy) is injected into and fills the cup, surrounding and encapsulating the LED. The encapsulant material is then cured to harden it around the LED. This packaging, however, can result in an LED package having significant variation of the color temperature of emitted light at different viewing angles with respect to the package. This color variation can be caused by a number of factors, including the different path lengths that light can travel through the conversion material. This problem can be made worse in packages where the phosphor containing matrix material extends above the “rim” of the cup in which the LED resides, resulting in a predominance of converted light emitted sideways into high viewing angles (e.g., at 90 degrees from the optic axis). The result is that the white light emitted by the LED package becomes non-uniform and can have bands or patches of light having different colors or intensities.
Another method for packaging or coating LEDs comprises direct coupling of phosphor particles onto the surfaces of the LED, also referred to as conformal coating. Electrophoretic deposition, screen printing, spin coating can be used to make conformal coating. These methods can result in improvement of the color uniformity as a function of viewing angle with one reason for this improvement being the source of the converted light and unconverted light being as close to the same point in space. For example, a blue emitting LED covered by a yellow converting material can provide a substantially uniform white light source because the converting material and LED are close to the same point in space. However, the color uniformity still suffers from phosphor concentration or phosphor thickness variations and differences in path length.
Inconsistencies in emission profile can also be the result of the angles of emission from an LED. FIG. 1a illustrates a cross-section of a known light emitting device or package. A light source, such as an LED 102, is disposed on a substrate/submount 104 and a layer of downconverting material 106 covers the LED 102. A reflector 108 is disposed around the LED 102 on the substrate 104 such that the LED 102 is housed in a cavity defined by the reflector 108 and the substrate 104, and a hemispherical encapsulant or lens 110 is disposed over the light source 102. The encapsulant 110 may be mounted over the light source 102 using an epoxy adhesive, for example, although other mounting methods may also be used. Light scattering particles 112 are disposed throughout the encapsulant 110.
It is noted that throughout the application reference is made to viewing angle and emission angles. The viewing angle is the angle at which an LED or LED package is viewed and is shown as exemplary θv in FIG. 1a. The viewing angle is measured from the optic axis which in this case runs through the center of the hemispherical encapsulant 110 and is perpendicular to the emitter 102. A viewing angle of zero degrees (0°) indicates that the output from the encapsulant is being viewed (or measured) from a point outside the encapsulant that is directly opposite the emitter 102, i.e., on-axis. The viewing angle increases as the device is tilted with respect to the viewer. A viewing angle of ninety degrees (90°) indicates that the output is being measured from an angle that is perpendicular to the optic axis and even with the flat edge of the encapsulant 110, i.e., directly from the side.
The emission angle is the angle at which light emits from an LED or LED package and is shown as θe in FIG. 1a. The emission angle shares the same optic axis with the viewing angle and it measures the angle from the optic axis at which a light ray initially propagates in the encapsulant 110 after it is emitted from the LED. A light ray that initially propagates from the emitter 102 along the optic axis (e.g., ray R1) has an emission angle of 0°. As shown ray θe is approximately forty degrees (40°). The emission angle increases as the direction of initial propagation deviates from the optic axis. An important difference between the two angles is that the output profile at a given viewing angle is affected by scattering events inside the encapsulant 110, whereas the emission angle describes the direction of the light as it is initially emitted from the source or LED before it can interact with materials within the encapsulant.
Light rays R1-R4 shown in FIG. 1a model the paths of exemplary photons that are emitted from the LED 102. As shown, R1 is emitted and passes through a length (l1) of the downconverting material 106 where there is a probability that a photon experiences a wavelength conversion. It is noted that the probability that a photon will be downconverted (i.e., absorbed and re-emitted) increases with the distance that the photon travels through the downconverting material 106. Thus, R2 which travels a greater distance (l2) through the downconverting material 106 has a greater chance of being downconverted. It follows that, depending on the shape of the downconverting layer, the percentage of light that experiences a downconversion upon passing through the downconverting layer 106 is a function of the angle of emission from the source 102.
Without light scattering particles, the emission spectrum can exhibit a non-uniform emission pattern, producing a emission with variances in color temperature and intensity often noticeable to the human eye. Such non-uniformities can render a light emitting device undesirable for certain applications. After passing through the downconverting material 106, the light enters the encapsulant 110. The light scattering particles 112 distributed throughout the encapsulant 110 are designed to redirect the individual photons before they are emitted to randomize the point where the photons exit the encapsulant 110. This has the effect of improving spatial color temperature uniformity. For example, R1 collides with a light scattering particle, changes direction, and is emitted as shown. R1 exits the encapsulant 110 at a different point than it would have if no scattering particles were present. R3 experiences multiple scattering events. R2 and R4 pass through the encapsulant unimpeded. Thus, the light scattering particles randomize (to a certain degree) the point at which emitted photons exit the encapsulant 110 by disassociating the photons from their initial emission angle.
LED packages can also be provided with multiple LEDs and often the LEDs in the same package can emit different colors. For example, red, green and blue emitting LEDs can be used in combination to form a white light package (solid state RGB). Other multi-chip combinations are also common, such as the solid state RGGB which comprises one red chip, one blue chip and two green chips per unit. Phosphor conversion layers may be used in conjunction with these multi-chip devices, for example, the phosphor converted RGB which is used for high Color Rendering Index applications. Another known device consists of a phosphor converted white LED and a solid state red chip. Other combinations of phosphor converted colored chips and solid state chips are also known in a multi-chip LED package. Scattering particles can also be used in multiple chip LED packages to help mix the different colors of light.
Light scattering particles, however, can also reduce the LED package's emission efficiency. Scattering particles of varying sizes are typically spread throughout the lens, such that many different wavelengths of light are scattered, even some of which do not need scattering. This can result in unnecessary backscattering and absorption of the light that need not be scattered, which can result in unacceptable losses in emission efficiency.